Single longitudinal mode operation of semiconductor lasers is a desirable property for many applications. Fundamental transverse mode operation has already been obtained using a variety of lateral confinement laser configurations. Single longitudinal mode operation has been obtained with distributed feedback (DFB) structures without the need of a Fabry-Perot cavity resonator. Lasers of this type are described in U.S. Pat. No. 3,675,157.
Because the spatial period of the DFB mechanism is half the optical guide wavelength, the fabrication of DFB lasers is quite difficult. In addition, the threshold currents generally are much higher than for lasers with cavity resonators.
Another form of longitudinal mode control was disclosed by P. J. deWaard in his paper entitled "A Novel Single Mode Laser Having Periodic Variations in the Stripe Width ("Super DFB")," which was presented at the Optical Communications Conference in Amsterdam, Netherlands, Sept. 17-19, 1979. In the laser described by deWaard, a combination of cavity resonator and a small amount of distributed feedback is used. To provide the DFB, the electrode contains three periodic variations in the width of the electrode as a means of controlling longitudinal mode behavior. Unlike the typical DFB configuration, however, the period of the width variations is many multiples of half the optical wavelength. This relatively large variation in the dimension of the electrode has the advantage of simlifying the fabrication process.
An improvment over the deWaard laser is described in the copending application by I. P. Kaminow, Ser. No. 285,253, filed July 20, 1981, now U.S. Pat. No. 4,400,813 issued on Aug. 23, 1983. In the Kaminow laser, longitudinal mode stabilization is achieved by notching the ridge portion of a ridge waveguide laser at periodic intervals along its entire length. Electrical contact is made only at the topmost portions of the ridge. By pumping the laser at periodically spaced regions, only those longitudinal modes whose amplitude maxima conicide with the spatial distribution of the pumped regions are supported.
It has been found, however, that one of the reasons the mode control obtained by the above-described techniques is relatively weak is that the mode selective mechanism is located too far from the active region of the laser so that the high spatial frequency components of the periodic perturbation are very weak in the active region. A second reason is that these prior art devices depend on periodic gain variations and require large variations in gain in order to operate successfully. Since the maximum available gain is limited, approximately half the laser length must be weakly excited which, in turn, tends to limit the overall gain of the laser.
In U.S. Pat. No. 4,257,011 distributed feedback is obtained by means of a Bragg diffraction grating which introduces variations in the complex refractive index of the wavepath. However, the desired spatial period of the grating, as disclosed in this patent, is as low an order as is permitted by the fabrication process. For example, in the illustrative embodiment described, for a 0.83 .mu.m laser, the order is three.